Kindle Unlimited is a service from Amazon that lets you "check out" (store on your Kindle or other e-reader with Kindle software) up to 10 e-books at a time. It costs $9.99 per month and is often available with promotional pricing for a try out. Here is an article with more details. (The article's website seems pretty good too.)
The books available through Kindle Unlimited are a smaller set of books than all e-books on Amazon and change regularly. The linked article says there were 1.4 million books as of December 2017.
I use it to explore new areas, and/or trashy, guilty pleasure books that I read in one day and return. I think it is worth it, but your mileage may vary.